The present invention relates to a method of fractionating mixtures of polymers, the solvents used for fractionation, and the fractionated polymers obtained thereby, which are useful for the formation of ferroelectric polymer films.
Ferroelectrics are a class of dielectric materials that can be given a permanent electric polarization by application of an external electric field. Use of ferroelectric materials in data processing devices is disclosed in U.S. Patent Application No. 2002/0044480 to Gudesen et al., which is directed to a ferroelectric data processing device comprising a thin film of ferroelectric material as a data-carrying medium. The film may be inorganic, a ceramic material, a polymer, or a liquid crystal. Gudesen does not, however, disclose how the ferroelectric films are made. Use of ferroelectric polymers in data processing devices is also described, for example, by Y. Tajitsu et al., in “Investigation of Switching Characteristics of Vinylidene Fluoride/Trifluoroethylene Copolymers in Relation to Their Structures”, (Japanese Journal of Applied Physics, Volume 26, pp. 554-560, 1987).
It is known that only certain vinylidene fluoride polymers are ferroelectric, and the presence of ferroelectricity is due at least in part to the history of the film, including the thermal history of the film and the solvent used to form the film. See, e.g., the Abstract of an article by Cho, in Polymer, Volume 15, p. 67 (1991). Recently, Tashiro et al., in Macromolecules, Volume 35, p. 714 (2002) performed a detailed structural analysis of the various vinylidene fluoride crystal morphologies. Vinylidene fluoride polymers occur in four distinct crystal morphologies, all monoclinic. Without intending to bound by theory, form I has essentially planar zigzag chains forming a polar structure in which CF2 dipoles are parallel to each other along the crystallographic b-axis. The chains are tightly packed and tend to form large crystals. In form II, the CF2 dipoles are packed in anti-parallel mode along the b-axis. Form II is therefore nonpolar and less tightly packed than form I. Form III is also a tightly packed polar unit cell, and is obtained by casting from highly polar (but not necessarily hydrogen bonding) solvents such as dimethylacetamide or dimethylformamide. Form III may also be obtained by annealing forms II or IV at high temperature. Finally, form IV is a polar structure in which the chains are packed in parallel mode. Form IV is also a desirable form from the standpoint of ferroelectric properties because it can interconvert with form II. Copolymers of vinylidene fluoride exhibit similar characteristics.
In addition to ferroelectricity, a number of other properties are important in the function and use of ferroelectric polymer films, including properties related to hysteresis (including saturation potential, coercive field strength, and permittivity); reliability (such as fatigue, aging, time dependence dielectric breakdown, imprint, and relaxation); and thermodynamic properties such as the Curie transition temperature of the film. Many factors can affect these properties, for example the composition of the ferroelectric polymers, and historical factors such as the heat load applied to a particular film, the process of by which a film is made, the solvent used to make a film, and mechanical stresses applied to the film. While there have been several attempts to improve the techniques used to form vinylidene fluoride films, there has been less emphasis on controlling the composition of the vinylidene fluoride polymer itself.
Typical synthetic procedures for vinylidene fluoride polymers yield mixtures of polymers with differences among these attributes. For example, Furukawa et al., in Adv. Colloid and Interface Sci., Vol. 71-72, p. 183 (1997), discusses the relationship between polymer composition of vinylidene fluoride-trifluoroethylene copolymers, and Curie temperature, the temperature at which a transition between the ferroelectric and paraelectric state occurs, finding that Curie temperature is very sensitive to composition. In addition, composition affects the full-scale displacement in polarization of ferroelectric vinylidene fluoride-hexafluoropropylene copolymers, as discussed by Ambalangodage et al. in Appl. Surf. Sci/Vol. 175-176, pg. 386 (2001).
U.S. Pat. No. 4,946,913 to Kappler is directed to controlling the molar composition and molecular weight of ferroelectric vinylidene fluoride and trifluoroethylene copolymers by adjustment of the reaction conditions for forming the polymers (i.e., rate of addition monomer addition, time, temperature, pressure, “protection colloid” and reaction initiator). However, there appear to be no reports of methods for adjusting or optimizing the molecular weight distribution of ferroelectric polymers in particular.
General methods for adjusting molecular weight distributions include processes such as chromatographic separation, and mixed solvent fractionation. As described in “Polymer Fractionation”, (Springer-Verlag, (1994), Chapter 4, by Francuskiewicz), in mixed solvent fractionation a solution of a dissolved polymer is treated with solvent-miscible polymer precipitant (also referred to as an anti solvent or a non-solvent) thereby causing a portion of the polymer to separate from the solution. These fractions of precipitated polymers may then be removed consistent with the desired outcome. Three factors that should be considered in selection of both the solvent and the precipitant include the solubility strength of the solvent, the precipitation strength of the precipitant, and the miscibility of the solvent and the precipitant. According to Francuskiewicz, neither the solvent nor the precipitant should be too strong, the solvent-precipitant system should display a high sensitivity to temperature, and the system should have a lower density than the polymer. However, the inventors hereof have found that these factors alone are insufficient to provide effective methods for the fractionation of vinylidene fluoride polymers such that the result is a narrow molecular weight distribution and a specific composition.
U.S. Pat. No. 5,264,536 to Radosz discloses use of mixed solvent fractionation of polymer mixtures to further narrow the molecular weight distribution of polyolefins beyond that achievable by mere control of reaction conditions. Fractionation of ferroelectric polymers is not disclosed. This method, however, requires use of supercritical solvents, which has practical limitations. Further, neither method has been shown to be applicable to vinylidene fluoride polymers, particularly ferromagnetic vinylidene fluoride polymers. Accordingly, there remains a need for economical, highly reproducible methods for the manufacture of ferroelectric polymers having a narrow molecular weight distribution, and in particular a weight distribution that allows the production of films, in particular films suitable for use as memory devices.